This invention relates to a means of guiding a circuit board which is to be tested into accurate positioning on a test location on a test fixture. It is particularly intended for use with test fixtures of the bed of nails type, and with automatic and robotic means of placement of the board.
Bed of nails test fixtures are widely used in the electronics industry for the electrical testing of printed circuit boards, both bare and loaded. The board is brought into electrical contact with an array of test probes which are placed so as to contact predetermined test points on the board. Testing of the entire circuit, or of the individual circuits of components, can be carried out using methods known to those skilled in the art.
In order for the test results to be accurate, the test probes must accurately contact the corresponding test points on the board. As modern board design produces increased circuit and component density, the required degree of accuracy of contact is also increased.
In almost all commercially available test fixtures in present use, accurate positioning of the printed circuit board to be tested over the test probe array is achieved by the use of tooling guide pins fixed onto the test fixture. Most printed circuit boards are supplied with two or more tooling holes as part of the process of manufacture, and for such boards the tooling guide pins are placed so that the board is correctly located when the tooling guide pins are located in the board's tooling holes. For boards which do not have tooling holes, tooling guide pins located so as to contact the edges of the board may be used.
It is obvious that, the better the fit between the tooling guide pin and the tooling hole, the more accurate the placement of the board will be. Conversely, the more accurate this fit, the more difficult it is to place the board onto the tooling guide pins. It has been for several years the common experience of those in the field that tooling guide pins often became bent or demaged because of misplacement. Also, the need for accurate placement means that the process takes more time, and the operator tires more quickly.
The common prior art solution to this problem was to use large diameter tooling guide pins, and to make the tooling holes of a diameter significantly larger than the diameter of the guide pin. However, while having the tooling hole larger than the guide pin made it easier to locate the board over the guide pins, the resulting play meant loss of positioning accuracy. Also, with the trend to increasing circuit and component density on circuit boards, the space available on the boards for tooling holes is limited. Another prior art solution was to taper the tips of the guide pins, thus making insertion of those pins into the tooling holes a little easier. This meant, however, that the tips of the pins were weakened and more easily damaged.
Further problems were encountered by those in the field when attempts were made to automate the testing process. In order to align the tooling holes with the tooling guide pins, alignment in all six degrees of freedom, that is left/right, back/forward, up/down and pitch, yaw and roll, must be achieved with a high degree of repeatability and accuracy. This is extremely difficult for a robot to achieve.